The current consumption of a mobile station when operating in an idle mode has a static component and a dynamic component. The static power consumption mainly results from the operation of the display, the crystal oscillator, a quiescent current consumed by voltage regulators, etc. The dynamic power consumption is, on the other hand, caused by the reception of messages, the transmission of messages, and the required signal processing (both analog and digital) relating to these tasks. A major portion of the dynamic power consumption is caused by the operation of the receiver and related circuitry during the reception of messages from a serving cell, and by neighbor cell monitoring used mainly for cell reselection purposes.
When a mobile station is powered on, and ready to receive or originate a call, the phone can be considered to be in a standby mode of operation. In this mode the mobile station will typically monitor a paging channel to detect the occurrence of an incoming call. For a battery powered mobile station it is important that the power consumption be minimized when in the standby mode in order to reduce battery drain. Preferably, one or more power saving techniques are employed, such as placing the mobile station's circuitry into a low power or quiescent mode, and only periodically reactivating the circuitry in order to receive and check the paging channel for an incoming call.
The following U.S. Patents are representative of various power saving techniques: U.S. Pat. No. 4,777,655, entitled "Control Signal Receiving System", by Numata et al.; U.S. Pat. No. 4,903,319, entitled "Portable Radio Telephone Having Power Saver", by Kasai et al.; U.S. Pat. No. 5,027,428, entitled "Power Saving Arrangement and Power Saving Method", by Ishiguro et al.; U.S. Pat. No. 5,031,231, entitled "Mobile Telephone Station with Power Saving Circuit", by Miyazaki; U.S. Pat. No. 5,265,270, entitled "Method and Apparatus for Providing Power Conservation in a Communication System, by Stengel et al.; and U.S. Pat. No. 5,293,693, entitled "Reduction of Power Consumption in a Portable Communication System". Reference in this regard can also be had to commonly assigned U.S. Pat. No. 5,471,655, entitled "Method and Apparatus for Operating a Radiotelephone in an Extended Stand-by Mode of Operation for Conserving Battery Power", by Raimo Kivari.
In one mobile station air interface standard that employs a Digital Control Channel (DCCH), known as IS-136.1, Rev. A, neighbor channel signal measurements for a DCCH reselection procedure consume much of the mobile station standby power. If the necessity to perform the neighbor channel measurements were eliminated the standby time could be increased. However, as currently specified the standard has a mandatory set of rules for neighbor channel measurements. These rules have been designed so as to maintain the mobile station tuned to a best available DCCH channel.
The IS-136 standard permits the mobile station to double the measurement period for any neighbor channel if, after several minutes, the channel is found to be stable (i.e., the rate of change of signal strength of the measured channel is found to be below some threshold value). The serving DCCH channel, however, is required to be measured at given, fixed intervals.
More particularly, and as is specified in IS-136.1, Rev. A, Mar. 21, 1996, Section 6.3.3.1, several information elements are included in the control channel Selection Parameters message sent on a Fast Broadcast Control Channel (F-BCCH), in a Neighbor Cell message, and in a Neighbor Cell (Multi Hyperband) message sent on an Extended Broadcast Control Channel (E-BCCH). These information elements are used in determining the interval of signal strength measurements (measurement interval) for the serving DCCH, for the Neighbor List (NL) entries, for Private Operating Frequencies (POFs, see Section 6.3.21), and for any DCCH identified as a result of a Non-Public Mode Search (see Section 6.3.19), as follows.
SCANINTERVAL: This information element represents the basic measurement interval in Hyperframes that is to be used for each frequency identified as requiring signal strength measurements. A Hyperframe consists of two Superframes, and has a total duration of 1.28 seconds.
HL-FREQ: There is one instance of this information element for each entry in the NL. HL-Freq is used to modify the SCANINTERVAL for each entry in the NL as follows. If HL-FREQ is equal to HIGH, the measurement-interval for the associated NL entry is given by SCANINTERVAL. If HL-FREQ is equal to LOW, the measurement interval for the associated NL entry is twice SCANINTERVAL. However, a mobile station may choose to measure all frequencies as if they have HL-FREQ set to HIGH.
Whenever there is a change in either the SCANINTERVAL or the NL contents, the mobile station computes a measurement interval for each entry in the NL. The measurement interval for the serving DCCH, for the Private Operating Frequencies (POFs), and for any DCCH identified as a result of a Non-Public Mode Search (NPS-DCCH), are always set to SCANINTERVAL. A mobile station's current Paging Frame Class (PFC) does not influence the computation of the measurement interval.
The mobile station measures the signal strength of the serving DCCH, POFs, NPS-DCCH and all viable NL entries each time their associated measurement interval lapses, and process the results according to a Reselection Criteria procedure (see Section 6.3.3.4). A given NL entry is considered viable if it is in a Hyperband supported by the mobile station; if it has a Network Type supported by the mobile station; or if it uses a form of modulation supported by the mobile station (i.e., FSK, and/or .pi./4-DQPSK).
As an example, for the case where SCANINTERVAL=0 and the NL contains eight entries having HL-FREQ=1, and eight entries having HL-FREQ=0, the following would result. For the serving DCCH, the measurement interval is equal to the period of one Hyperframe (i.e., 1.28 seconds). For NL entries having HL-FREQ=1, the measurement interval is also equal to one Hyperframe. For NL entries having HL-FREQ=0, the measurement interval is equal to two Hyperframes, or 2.56 seconds. The total number of signal strength measurements made per Hyperframe is thus (1+8+8)/2=13 (including the serving DCCH).
As is indicated in Section 6.3.3.2, if the value of a Scanning Option Indicator information element sent in a Control Channel Selection Parameters message is set to zero, the mobile station is not permitted to support any optional enhancements to the measurement interval. If the value of the Scanning Option Indicator information element is set to a one, the mobile station has the option of increasing the measurement interval (see Section 6.3.3.1) for one or more of the NL entries. There are three conditions that a mobile station can detect and respond to by increasing the measurement interval. A mobile station may increase the measurement interval, for any given NL entry, multiple times should more than one of these conditions be coincidental. The three conditions that may result in a mobile station increasing the measurement interval for one or more NL entries are as follows.
First, if the time since the last control channel reselection is greater than one hour, then the mobile station is allowed to increase the measurement interval for all NL entries by a factor of two.
Second, the mobile station must first make 25 signal strength measurements on the serving DCCH and NL entries in order to produce a valid processed signal strength (PSS) value. The algorithms used to produce PSS values are specified to consist of either a linear average (current measurement value plus 24 previous measurement values divided by 25), or of an exponential average ((current meas./25)+previous PSS*24/25). If the change in PSS on the serving DCCH is less than 7 dB over the last five minutes, and if the change in the PSS on all NL entries is less than 7 dB over the last five minutes, a mobile station is allowed to increase the measurement interval for all NL entries by a factor of two.
Third, if the difference between the PSS on the serving DCCH and the PSS for a NL entry is less than 10 dB over the last five minutes, a mobile station is allowed to increase the measurement interval for that NL entry by a factor of two.
As soon as a condition is no longer valid, the mobile station revokes the corresponding increase in measurement interval for all affected NL entries.
As is specified in Section 6.3.3.3, the mobile station must keep a running average of the last five signal strength measurements of the current DCCH (Long.sub.-- RSS) for each measured frequency. Additionally, the mobile station must keep a running average of the last two signal strength measurements of the current DCCH (Short.sub.-- RSS). Both of these values are then used for the control channel reselection procedure (see Section 6.3.3.4). The interval of signal strength measurements for any given frequency is determined by the value of the measurement interval, as was explained above in the discussion of Sections 6.3.3.1 and 6.3.3.2.
After the mobile station camps on a control channel a Full.sub.-- reselect.sub.-- data.sub.-- indicator is reset. After collecting five signal strength measurements for each viable Neighbor List entry, the Full.sub.-- reselect.sub.-- data.sub.-- indicator is set to show that valid average Long.sub.-- RSS values are available, and that Neighbor List control channels can be considered for reselection purposes.
Reference may also be had to Sections 6.3.3.4 (Reselection Criteria), 6.3.3.4.1 (Reselection Trigger Conditions (RTC)), and 6.3.3.4.2 (Candidate Eligibility Filtering (CEF)).
As should be apparent from the foregoing discussion of one exemplary air interface standard, a mobile station can reduce but not eliminate the requirement to make signal strength measurements of channels besides the serving DCCH. As such, and although a reduction in battery power consumption can be realized, the reduction is not sufficient to maximize the amount of standby time that can be achieved before the mobile station's battery must be recharged.
Although the foregoing discussion has been presented in the context of an IS-136 based system, similar problems are present in other air interfaces. By example, in the GSM system neighbor cell monitoring involves making received signal strength (RSSI) measurements, performing Base Station Identity Code (BSIC) monitoring of the "best" neighboring cells (e.g., the six best neighbor cells), and receiving those BCCH parameters which control cell reselection. Before monitoring the BSIC of the neighbor cell a synchronization must be reached with the cell by the FCCH. Although neighbor cell monitoring can be affected by the parameters sent by the network, the neighbor cell monitoring is usually designed so as to be performed even in small cells and when the velocity of mobile station is high. In many situations, however, the mobile station is not moving, or is moving relatively slowly with respect to the neighboring cells.
In this regard it is noted that the reception operations of the mobile station may be divided in two categories: serving cell operations and adjacent cell operations.
The serving cell operations include the paging channel (PCH) reception, the broadcast control channel (BCCH) reception, and the cell broadcast channel (CBCH) reception, none of which can avoided.
The adjacent cell operations include neighbor cell synchronization channel (SCH) reception, neighbor cell system parameter acquisition from the neighbor cell BCCH, and the RSSI measurement of the neighboring cells. However, the sole purpose of these operations is to support the cell reselection procedures, which are required, typically, only when the mobile station is moving.